Behari explained it succinctly (hard to make long arguments when you are screaming in rage). The guy gave her hope that she had already abandoned, and then took it away one day at a time for his inaction.

But... he was a poor kid forced into mercenary service, a rookie who had one lucky shot and was poorly trained and ill-prepared to mount a rescue... would love had conquered all had he tried to steal Behari from the brothel? Would he have ended up beaten up in a curb somewhere, Behari harshly punished?

Al-X Melchor"Fiction is written by a single man; myth is written by an entire culture."-ReGenesis

We first see her dressed in rags and physical contact triggers a violent defensive response. We later learn that she was forced into sexual slavery under false pretenses, and that she was deceived into thinking her own own father had sold her. Hell, her psychopomp wants to mercy-kill her since she's had such a crap life.

The kid's ego writes a check his body can't cash, and abandons her without demonstrating the integrity to confess that there's no way he can afford to purchase her contract. That is a betrayal, and one all the more severe because, again, Behari is a slave, with no ability to give or withhold her consent, and he slept with her. That gives him full complicity in the injustice that's taken place against her.

Finally, he doesn't make any other attempt to intervene or assist. Even as a mercenary apprentice, he has more freedom than Behari did, enough to move around town unescorted. He could have investigated the contract or Behari's family or sought out a lawyer or scribe (or in-culture equivalent) for aid. If he can afford to spend a night carousing at a brothel he can afford to pay a lawyer to look at a contract for five minutes. Derexxi was able to pull apart the contract as bogus within seconds of looking at it, and the brothel owner leaves immediately, without protest, after realizing that his contract has no legal force. Even though Derexxi is far more worldly and experienced, the scene demonstrates conclusively that there were other available options.